Concentrating Solar Power (CSP) systems utilize solar energy to drive a thermal power cycle for the generation of electricity. CSP technologies include parabolic trough, linear Fresnel, central receiver or “power tower”, and dish/engine systems. Considerable interest in CSP has been drive by renewable energy portfolio standards applicable to energy providers in the southwestern United States and renewable energy feed-in tariffs in Spain. CSP systems are typically deployed as large, centralized power plants to take advantage of economies of scale. A key advantage of certain CSP systems, in particular parabolic troughs and power towers, is the ability to incorporate thermal energy storage. Thermal energy storage (TES) is often less expensive and more efficient than electric storage and allows CSP plants to increase capacity factor and dispatch power as needed—for example, to cover evening or other demand peaks.
Current CSP plants typically utilize oil, molten salt or steam to transfer solar energy from a solar energy collection field, tower or other apparatus to the power generation block. These fluids are generally referred to as a “heat transfer fluid” and are typically flowed through a heat exchanger to heat water to steam or to heat an alternative “working fluid” which is then used to drive a turbine and generate electrical power. Commonly utilized heat transfer fluids have properties that in certain instances limit plant performance; for example, synthetic oil heat transfer fluid has an upper temperature limit of 390° C., molten salt has an upper temperature limit of about 565° C. while direct steam generation requires complex controls and allows for limited thermal storage capacity.
Current state-of-the-art two-tank molten salt storage costs are relatively high, and impose temperature limitations upon a practical system. For example, a typical two-tank molten salt storage system will freeze at temperatures under 200° C. and become unstable above 600° C. Proposed single-tank thermocline TES systems have the potential to displace 75% of the expensive molten salt with low cost rocks or pebbles. Even so, the cost of a thermocline TES system will still be high due to the cost of the remaining 25% salt or other required elements such as a stainless steel tank. In addition, molten salt may still limit the highest operating temperature of the overall CSP system for the power generation and thereby limit system efficiency. In addition, TES salt transportation and conditioning can take several months, which negatively impacts capital investment.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.